In this zone of the hearth, the fluidized solids subject the coating to high levels of stress by erosion because of the high speeds imparted to them by the fluidization gas being ejected. Moreover, because of the large temperature differences between the inside of the hearth and the fluidization gas at the outputs of the injection nozzles via which the gas is injected, this zone of the hearth is subjected to high levels of thermal stress.
It is known that the supporting sheet can be protected by a layer of refractory material of the refractory concrete type.
Such a layer must be very thick, i.e. greater than 100 mm. That thickness increases both the total height of the furnace, and also the lengths of the feed means for feeding the fluidization nozzles.
Such a layer is also subjected to continuous wear and tear which makes it necessary for periodic repair work to be performed. At floor level in the hearth, the concentration of solids is very high, i.e. in the approximate range 500 kg/m.sup.3 to 1,200 kg/m.sup.3.
Furthermore, with wear, chips of the layer come away and mix in with fluidized combustion solids, thereby disturbing fluidization.